想必大家對這一段JAVA代碼一定不會陌生：

public classTest {public static voidmain(String[] args) {

System.out.println("Hello World!");

}

}

輸出：Hello World!

今天咱們就從頭開始分析一下它是如何從編譯到輸出的。

1、編譯

javac Test.java，生成JAVA的字節碼文件：Test.class。使用hexdump -C Test.class查看如下：

00000000 ca fe ba be 00 00 00 32 00 1d 0a 00 06 00 0f 09 |.......2........|

00000010 00 10 00 11 08 00 12 0a 00 13 00 14 07 00 15 07 |................|

00000020 00 16 01 00 06 3c 69 6e 69 74 3e 01 00 03 28 29 |........()|

00000030 56 01 00 04 43 6f 64 65 01 00 0f 4c 69 6e 65 4e |V...Code...LineN|

00000040 75 6d 62 65 72 54 61 62 6c 65 01 00 04 6d 61 69 |umberTable...mai|

00000050 6e 01 00 16 28 5b 4c 6a 61 76 61 2f 6c 61 6e 67 |n...([Ljava/lang|

00000060 2f 53 74 72 69 6e 67 3b 29 56 01 00 0a 53 6f 75 |/String;)V...Sou|

00000070 72 63 65 46 69 6c 65 01 00 09 54 65 73 74 2e 6a |rceFile...Test.j|

00000080 61 76 61 0c 00 07 00 08 07 00 17 0c 00 18 00 19 |ava.............|

00000090 01 00 0c 48 65 6c 6c 6f 20 57 6f 72 6c 64 21 07 |...Hello World!.|000000a000 1a 0c 00 1b 00 1c 01 00 04 54 65 73 74 01 00 |..........Test..|000000b010 6a 61 76 61 2f 6c 61 6e 67 2f 4f 62 6a 65 63 |.java/lang/Objec|000000c074 01 00 10 6a 61 76 61 2f 6c 61 6e 67 2f 53 79 |t...java/lang/Sy|000000d073 74 65 6d 01 00 03 6f 75 74 01 00 15 4c 6a 61 |stem...out...Lja|000000e076 61 2f 69 6f 2f 50 72 69 6e 74 53 74 72 65 61 |va/io/PrintStrea|000000f0 6d 3b01 00 13 6a 61 76 61 2f 69 6f 2f 50 72 69 |m;...java/io/Pri|

00000100 6e 74 53 74 72 65 61 6d 01 00 07 70 72 69 6e 74 |ntStream...print|

00000110 6c 6e 01 00 15 28 4c 6a 61 76 61 2f 6c 61 6e 67 |ln...(Ljava/lang|

00000120 2f 53 74 72 69 6e 67 3b 29 56 00 21 00 05 00 06 |/String;)V.!....|

00000130 00 00 00 00 00 02 00 01 00 07 00 08 00 01 00 09 |................|

00000140 00 00 00 1d 00 01 00 01 00 00 00 05 2a b7 00 01 |............*...|

00000150 b1 00 00 00 01 00 0a 00 00 00 06 00 01 00 00 00 |................|

00000160 01 00 09 00 0b 00 0c 00 01 00 09 00 00 00 25 00 |..............%.|

00000170 02 00 01 00 00 00 09 b2 00 02 12 03 b6 00 04 b1 |................|

00000180 00 00 00 01 00 0a 00 00 00 0a 00 02 00 00 00 03 |................|

00000190 00 08 00 04 00 01 00 0d 00 00 00 02 00 0e |..............|0000019e

這是二進制文件，看不明沒有關系，java提供了javap可以查看內容是什么。

2、查看CLASS文件

Compiled from "Test.java"

public class Test extendsjava.lang.Object

SourceFile:"Test.java"minor version:0major version:50Constant pool:const #1 = Method #6.#15; //java/lang/Object."":()V

const #2 = Field #16.#17; //java/lang/System.out:Ljava/io/PrintStream;

const #3 = String #18; //Hello World!

const #4 = Method #19.#20; //java/io/PrintStream.println:(Ljava/lang/String;)V

const #5 = class #21; //Test

const #6 = class #22; //java/lang/Object

const #7 = Asciz ;const #8 =Asciz ()V;const #9 =Asciz Code;const #10 =Asciz LineNumberTable;const #11 =Asciz main;const #12 = Asciz ([Ljava/lang/String;)V;const #13 =Asciz SourceFile;const #14 =Asciz Test.java;const #15 = NameAndType #7:#8;//"":()V

const #16 = class #23; //java/lang/System

const #17 = NameAndType #24:#25;//out:Ljava/io/PrintStream;

const #18 = Asciz Hello World!;const #19 = class #26; //java/io/PrintStream

const #20 = NameAndType #27:#28;//println:(Ljava/lang/String;)V

const #21 =Asciz Test;const #22 = Asciz java/lang/Object;const #23 = Asciz java/lang/System;const #24 =Asciz out;const #25 = Asciz Ljava/io/PrintStream;;const #26 = Asciz java/io/PrintStream;const #27 =Asciz println;const #28 = Asciz (Ljava/lang/String;)V;

{publicTest();

Code:

Stack=1, Locals=1, Args_size=1

0: aload_01: invokespecial #1; //Method java/lang/Object."":()V

4: returnLineNumberTable:

line1: 0

public static voidmain(java.lang.String[]);

Code:

Stack=2, Locals=1, Args_size=1

0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;

3: ldc #3; //String Hello World!

5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

8: returnLineNumberTable:

line3: 0line4: 8}

別看寫的挺多的，分析一下么有什么：

然后const的部分是將內容放入內存

public Test()是默認的構造函數

有用的就是：public static void main() .............................

如果執行java Test，那么

3、申請內存棧

4、輸出內容

這方面咱們比較關心，查看代碼：

3: ldc #3; //String Hello World!

這句話的意思是：將字符串Hello World!放入棧頂

invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

這句話的意思是輸出內容：Hello World!

總結

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